Changes in the NMDA receptor concentration are thought to be involved in ischemia, Alzheimer's disease and Parkinson's disease. Two radiopharmaceuticals that bind to the NMDA receptor have been prepared: [11C] labeled MK 801 and [18F] labeled fluorothicnylcyclohexylpiperdine. Both compounds have been synthesized in good chemical yield. Other derivatives such as hydroxyphencyclidine are now being radiolabeled. The tritiated form of the compounds has been tested and the relative affinities for the NMDA receptor are in the order MK-801,TCP,PCP=FTCP.